JP4775036B2 - Transmission bandwidth allocation method and apparatus - Google Patents

Description

  The present invention relates to a transmission band allocation method and apparatus in an optical transmission system using TDMA.

  An optical access method using GE-PON (Gigabit Ethernet Passive Optical Network) is becoming widespread. Ethernet and Ethernet are registered trademarks. In the PON, a plurality of user optical terminators ONU (Optical Network Unit) are connected to an optical terminator OLT (Optical Line Terminal) arranged in a center station via an optical transmission line composed of an optical fiber and an optical coupler. GE-PON generally accommodates 16 or 32 ONUs with one optical fiber, but there is no upper limit to the number of ONUs that can be accommodated with one optical fiber, and 64 or 128 is standard. It is also possible to accommodate individual ONUs.

  In GE-PON, TDMA (Time Domain Multiple Access) is used for transmission of an upstream signal from an ONU. That is, the OLT notifies each ONU of the transmission timing and transmission period, and the ONU transmits a signal within the transmission period of the given transmission timing.

  In current Internet communication, TCP is used for file download. In the TCP communication, as shown in FIG. 9, the server transmits a certain amount of data WS (Window Size) to the client, and the client returns an ACK signal as a reception notification to the server. When the server receives the ACK signal, the server transmits data next to the data amount WS to the client. These procedures are repeated until all data has been transmitted.

  As described above, the transmission of data of the data amount WS from the server to the client and the transmission of the ACK signal from the client to the server are set as a set, and the data is transmitted from the server to the client by repeating this. The time from when the server transmits data for WS until it receives the ACK signal from the client is called RTT (Round Trip Time). Since the data amount WS is transmitted at the time RTT, the TCP throughput is defined by WS / RTT.

  When GE-PON is interposed between the server and the client, TCP communication is realized by the procedure shown in FIG. (1) The ONU to which the client is connected temporarily stores the ACK signal from the client in the buffer. (2) This ONU notifies the OLT that there is data to be transmitted using a Report frame. (3) The OLT aggregates the Report frames from each ONU and notifies each ONU of the transmission timing and transmission period by using the Gate frame. (4) The ONU transmits an ACK signal to the server using the transmission timing and transmission period indicated by the Gate frame from the OLT.

In practice, in GE-PON, the above procedure is performed in units of LLID (Logical Layer Identifier), but here, it is described in a simplified manner. A plurality of LLIDs can be assigned to the ONU.
Japanese Patent Laid-Open No. 2005-012800

  When GE-PON is used, since the ACK signal is temporarily accumulated in the ONU, the TCP throughput RTT includes the transmission waiting time of the ACK signal in the ONU, which greatly deteriorates the TCP throughput.

  If the transmission interval assigned to each ONU by the OLT is shortened, the transmission waiting time can be shortened. However, conventionally, since the transmission period is constant for any ONU, if the transmission interval is shortened, the transmission period is shortened. When the transmission period is shortened, the overhead increases relatively, so that the effective uplink bandwidth decreases.

  FIG. 11 shows an example of the arrangement of transmission frames when the transmission interval is large and small. FIG. 11A shows a case where the transmission interval is small, and FIG. 11B shows a case where the transmission interval is large. 11 (a) and 11 (b), it can be seen that the smaller the transmission interval, the shorter the waiting time until the next transmission timing, but the overhead portion is relatively increased and the effective data rate is lowered.

  In the access system, downstream throughput is more important than upstream throughput.

  It is an object of the present invention to provide a transmission band allocation method and apparatus for improving downlink TCP throughput in the TDMA scheme.

The transmission band allocation method according to the present invention is a data transmission system in which a plurality of user termination devices are accommodated and each user termination device is permitted to transmit by TDMA, and a TCP that detects the presence or absence of downlink TCP communication to each user termination device. A transmission interval that is permitted to a user terminal device having the downlink TCP communication according to the detection result of the TCP communication detection step while maintaining the communication detection step and the transmittable data amount within a predetermined reference period of each user terminal device equal to each other And a control step for shortening the transmission bandwidth.

A transmission band allocation device according to the present invention is a data transmission system that accommodates a plurality of user termination devices and permits transmission to each user termination device by TDMA, and detects TCP presence / absence of downlink TCP communication to each user termination device The communication detection device, the TCP communication table updated by the detection result of the TCP communication detection device and indicating the presence / absence of downlink TCP communication of each user termination device, and the transmittable data amount within the predetermined reference period of each user termination device are mutually connected. A control device that shortens a transmission interval permitted to a user terminal device with the downlink TCP communication while referring to the TCP communication table while maintaining the same is provided.

  According to the present invention, the transmission interval of the ACK signal for downlink TCP communication can be shortened, and as a result, the throughput of downlink TCP can be improved.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 shows a schematic block diagram of an embodiment of the present invention. FIG. 2 shows a sequence from TCP connection to disconnection. Here, in order to facilitate understanding, a case will be described in which a personal computer connected to one of the ONUs in the PON system that accommodates three ONUs communicates with a server in the upper network. . As described above, in general, more ONUs can be connected to the GE-PON system.

  An uplink port 10a of an optical termination device (OLT) 10 disposed in the center station is connected to the upper network 12, and a server 14 is connected to the upper network 12. The interface between the uplink port 10a and the upper network 12 is 1000Base-T or 1000Base-SX.

  The PON port 10 b of the OLT 10 is connected to the 1: n optical coupler 18 through the optical fiber 16. In the illustrated example, n = 3. The optical coupler 18 divides the downstream signal light from the optical fiber 16 into three parts and outputs the divided signal lights to the optical fibers 20-1 to 20-3. The other ends of the optical fibers 20-1 to 20-3 are connected to optical terminal units (ONUs) 22-1 to 22-3 disposed in the respective user houses. Computers 24-1 to 24-3 are connected to the respective ONUs 22-1 to 22-3. Of course, a plurality of computers can be connected to one ONU via a router or a gateway.

  The OLT 10 controls the transmission timing and transmission period of each of the ONUs 22-1 to 22-3 and relays data transmission between the ONUs 22-1 to 22-3 and the upper network 12.

  Downlink data signals from the upper network 12 are input to the LAN interface 30 via the uplink port 10a. The LAN interface 30 outputs the downlink data signal to the multiplexing device 32. The OLT control device 34 that controls the OLT 10 receives various control signals for the ONUs 22-1 to 22-3 (for example, a Gate message that indicates the transmission timing and transmission period of each ONU 22-1 to 22-3). Output to.

  The multiplexing device 32 multiplexes the control signal from the OLT control device 34 on the downlink data signal from the network interface 30 and applies the multiplexed signal to the electrical / optical converter 36. The electrical / optical converter 36 converts the multiplexed signal from the multiplexer 32 into an optical signal (downstream signal light) and applies it to the WDM optical coupler 38. The WDM optical coupler 38 outputs the downstream signal light from the electrical / optical converter 36 to the optical fiber 16 via the PON port 10b.

  Downstream signal light output from the PON port 10b of the OLT 10 is input to each ONU 22-1 to 22-3 via the optical fiber 16, the optical coupler 18, and the optical fibers 20-1 to 20-3.

  Each of the ONUs 22-1 to 22-3 converts the downstream signal light input from the optical fibers 20-1 to 20-3 into an electric signal, takes it if it is addressed to itself, and discards it if it is addressed to another. Each of the ONUs 22-1 to 22-3 internally processes a control signal addressed to itself among the captured downstream signals, and sends data signals addressed to the subordinate computers 24-1 to 24-3 to the computers 24-1 to 24- 3 is supplied.

  On the other hand, the computers 24-1 to 24-3 output data signals directed to the upper network 12 to the ONUs 22-1 to 22-3, respectively. Each ONU 22-1 to 22-3 outputs upstream signal light to the optical fibers 20-1 to 20-n, respectively, at the transmission timing and transmission period permitted by the OLT 10. This upstream optical signal is a data signal (including a control signal and a response signal for the server 14 and the like) addressed to a device (for example, the server 14) connected to the upper network 12 from the subordinate computers 24-1 to 24-3. The ONUs 22-1 to 22-3 carry control signals output to the OLT 10. The control signal addressed to the OLT unit 10 includes a control signal for establishing a logical link, a response signal (for example, a Report message in response to a Gate message), and the like. The optical coupler 18 outputs upstream optical signals from the optical fibers 20-1 to 20-3 to the optical fiber 16. In this way, the upstream signal light output from each of the ONUs 22-1 to 22-3 is input to the WDM optical coupler 38 via the PON port 10 b of the OLT 10.

  The WDM optical coupler 38 supplies the upstream signal light from the PON port 10 b to the optical / electrical converter 40. The optical / electrical converter 40 converts the upstream signal light from the WDM optical coupler 38 into an electrical upstream signal. The separation device 42 supplies a signal addressed to the OLT 10 among the electrical upstream signals output from the optical / electrical converter to the OLT control device 34, and supplies an upstream signal addressed to the upper network 12 to the LAN interface 30.

  The LAN interface 30 outputs the upstream signal from the separation device 42 to the upper network 12.

  In this embodiment, the OLT control device 34 dynamically controls the transmission timing and transmission period of each ONU 22-1 to 22-3 in order to improve the downlink TCP throughput. For this purpose, the OLT 10 includes a TCP communication detection device 44 that monitors downlink TCP communication, a TCP communication table 46 that stores the status of downlink TCP communication, a timer 48 that updates the TCP communication table 46, and each ONU 22-1. A bandwidth allocation table 50 for storing uplink bandwidth allocation parameters for 22-3 is provided. That is, a TCP communication detection device 44, a TCP communication table 46, a timer 48, and a bandwidth allocation table 50 are added for dynamic control of transmission bandwidth allocation, and a transmission bandwidth allocation control function is incorporated in the OLT controller 34.

  The TCP communication detection device 44 intercepts a downstream signal supplied from the LAN interface 30 to the multiplexing device 32 to detect the presence or absence of TCP communication, and updates the TCP communication table 46 according to the detection result. FIG. 3 shows an example of the configuration and numerical values of the TCP communication table 46. The TCP communication table 46 includes an ONU (or LLID) number field 46a, a flag field 46b indicating presence / absence of downstream TCP communication, and a field 46c that stores a timer value that decreases with time.

  When detecting the downstream TCP communication, the TCP communication detecting device 44 sets a flag in the downstream TCP flag field 46b of the record of the corresponding ONU (or LLID) in the table 46, that is, sets '1' and the timer field 46c. Is set to an appropriate initial value (1 second in the example shown in FIG. 3). The OLT control device 34 refers to the output of the timer 48 and subtracts the timer value in the timer field 46 c of the table 46. When the timer value in the timer field 46c of the table 46 becomes 0, it does not decrease any more.

  When the TCP communication detecting device 44 detects a sign or symbol indicating the end of TCP communication in the downstream signal from the LAN interface 30 to the multiplexing device 32, the corresponding ONU (or LLID) record in the TCP communication table 46 is detected. The downstream TCP flag field 46b is cleared with '0', and the timer field 46c is cleared with 0.

  The timer value in the timer field 46c decreases with the passage of time when the TCP detection device 44 does not detect the downstream TCP communication. Specifically, the OLT control device 34 refers to the output of the timer 48 and decrements the numerical value in the timer field 46c of the table 46 with time.

  As will be described in detail later, when the timer value in the timer field 46c is not 0, the OLT control device 34 sets bandwidth allocation so that the downstream throughput is increased in the ONU that receives the downstream TCP communication. The set value of the timer field 46c is a margin for a case where the OLT 10 temporarily does not receive a downstream TCP frame due to an error in the upper network 12 or the like.

  A method in which the TCP communication detection device 44 detects downstream TCP communication will be described. In TCP communication, “00000110” (hexadecimal 0x06, decimal 6) is set in the “Protocol” portion of the IP header defined by RFC 791 of IETF. As can be seen from FIG. 2, when establishing a TCP connection, the transmission apparatus (for example, the server 14) sets “1” to the “SYN” bit in the “Control Bits” portion of the TCP header defined by RFC793. A TCP frame in which 'is set is transmitted to a receiving device (for example, any one of the computers 24-1 to 24-3). When disconnecting the TCP connection, the transmission apparatus transmits a TCP frame in which “1” is set in the “FIN” bit of the “Control Bits” portion of the TCP header to the reception apparatus. Therefore, the TCP communication can be determined by the “Protocol” portion of the IP header of the downstream signal, the start of the TCP communication can be determined by the “SYN” bit in the “Control Bits” portion of the TCP header, and the “Control Bits” of the TCP header. The end of TCP communication can be determined by the 'FIN' bit of the part.

  The presence / absence of downstream TCP communication can also be determined based on the presence / absence of data. That is, when the “Protocol” portion in the IP header is “00000110” (0x06 in hexadecimal and 6 in decimal) representing TCP, and the “ACK” bit in the “Control Bits” portion in the TCP header is “0” This indicates the presence of downlink data by downlink TCP communication. Since the ACK bit is used to indicate that a predetermined packet has been received, when the 'ACK' bit is '0', the downlink TCP frame is a frame carrying downlink data.

  When the TCP communication detection device 44 detects a frame indicating the start of TCP communication or detects a downstream TCP frame carrying data based on the value of the 'SYN' bit in the 'Control Bits' portion of the TCP header. The flag is set in the flag field 46b of the corresponding ONU (or LLID) record in the table 46, and a margin value (here, 1 second) is set in the timer field 46c. It may be simplified so that only one of the former and the latter is detected.

  The TCP communication detection device 44 also clears the downstream TCP flag field 46b of the TCP communication table 46 when detecting the end of the downstream TCP communication based on the value of the 'FIN' bit in the 'Control Bits' portion of the TCP header, 0 is set in the timer field 46c. As another method, the TCP communication detection device 44 may delete the record of the corresponding ONU (or LLID) in the table 46 when detecting the end of the downstream TCP communication.

  Considering a communication failure or the like, if the value of the timer field 46c becomes zero before detecting the disconnection of the downlink TCP communication, the OLT control device 34 considers that the downlink TCP communication is completed. As described above, the OLT control device 34 refers to the output of the timer 48 and decrements the timer value in the timer field 46c of the table 46 with time until it becomes zero.

  The OLT control device 34 refers to the TCP communication table 46 and updates the bandwidth allocation table 50 used for the uplink signal bandwidth allocation control to the ONUs 22-1 to 22-3. Then, the OLT control device 34 transmits the control signals from the ONUs 22-1 to 22-3 to the ONUs 22-1 to 22-3 according to the request for the transmission band and timing, for example, under the bandwidth allocation table 50. Assign timing and transmission period.

  A method for determining the bandwidth to be allocated will be described. FIG. 4 shows a numerical example of the bandwidth allocation table 50 when none of the ONUs 22-1 to 22-3 is performing downlink TCP communication. The table 50 includes a field 50a for designating an ONU (or LLID), a field 50b for designating a transmission interval, and a field 50c indicating an index for defining the maximum transmission data amount per time. In this example, the reference transmission interval is 1 ms.

  FIG. 5 shows an example of the contents of the table 50 after the TCP communication detecting device 44 detects the downstream TCP communication directed to the ONU 22-1. By detecting the downstream TCP communication for the ONU 22-1, the transmission interval for the ONU 22-1 is shortened from 1 ms to 0.5 ms. In this embodiment, since it is assumed that a fair transmission amount is ensured for the ONU that requires transmission, the transmission interval is halved as shown in the field 50c according to the fact that the transmission interval is halved. The maximum amount of transmitted data is also halved.

  The OLT control device 34 refers to the TCP communication table 46 and changes the bandwidth allocation table 50 to the ONU 22-1 (or LLID) as illustrated in FIG. Independent of the operation of changing the table 50, the OLT control device 34 refers to the bandwidth allocation table 50 and allocates the upstream signal bandwidth to each of the ONUs 22-1 to 22-3.

  When the bandwidth allocation table 50 has the contents shown in FIG. 4, the OLT control device 34 allocates a uniform maximum transmission data amount per time at equal transmission intervals to the ONUs 22-1 to 22-3. FIG. 6 is a schematic diagram of the timing sequence of the upstream signal on the optical fiber 16 corresponding to FIG.

  When the bandwidth allocation table 50 has the contents shown in FIG. 5, the OLT control device 34 follows the table 50 shown in FIG. 5, and sends the ONU 22-1 half the transmission interval of the other ONUs 22-2 and 22-3. And the maximum transmission data amount per time is half that of the other ONUs 22-2 and 22-3. The OLT control device 34 permits transmission of the ONU 22-1 that is increased by shortening the transmission interval at a timing that is appropriately dispersed within one period. FIG. 7 shows a schematic diagram of the timing sequence of the upstream signal on the optical fiber 16 corresponding to FIG. In the example shown in FIG. 7, a new transmission period to the ONU 22-1 is interrupted after the transmission period of the ONU 22-3 following the original transmission timing.

  In this embodiment, the transmission interval is described as it is in the field 50b of the table 50, but the number of transmissions within the reference period may be defined. For example, the reference period is set to 1 ms, and the number of transmissions within the one reference period is changed according to the throughput improvement target of the downlink TCP communication. When it is desired to equalize the amount of uplink transmission data that can be transmitted by all ONUs, the amount of data that can be transmitted per transmission is increased according to the number of transmissions so that the amount of data that can be transmitted within one reference period is maintained constant. Reduce. For ONUs having two or more transmission times, the transmission period is appropriately distributed within one reference period.

  The transmission period of the same length may be assigned more frequently according to the improvement target of the throughput of the downlink TCP communication. In this case, the transmission interval of other ONUs becomes long, and the throughput of downlink TCP decreases.

  In the embodiment shown in FIG. 1, the downlink signal is monitored, but the downlink TCP communication can be similarly detected by monitoring the uplink signal. FIG. 8 shows a schematic block diagram of an embodiment in which the presence / absence of downstream TCP communication is detected based on upstream signals. The same components as those in FIG. 1 are denoted by the same reference numerals. Instead of the TCP communication detection device 44, the OLT 60 is provided with a TCP communication detection device 44a that detects the presence or absence of the downstream TCP communication by intercepting the upstream signal.

  The operation of the TCP communication detection device 44a which is a changed part will be mainly described.

  The TCP communication detection device 44a intercepts the upstream signal output from the optical / electrical converter 40, and monitors the 'Control Bits' portion of the TCP header of the TCP frame in the upstream signal. As can be seen from FIG. 2, when establishing a TCP connection, the receiving device transmits a TCP frame in which the 'ACK' bit and the 'SYN' bit in the 'Control Bits' portion of the TCP header of the TCP frame are standing together. Send to device. Thereafter, data is transmitted from the transmission device to the reception device. When disconnecting the TCP connection, the receiving apparatus transmits a TCP frame in which the “FIN” bit is set in the “Control Bits” portion of the TCP header to the transmitting apparatus.

  The presence of downstream TCP communication can also be determined based on the presence or absence of data. That is, the 'Protocol' part in the IP header in the upstream signal is '00000110' (hexadecimal 0x06, decimal 10), and the 'ACK' bit in the 'Control Bits' part in the TCP header is '1' In the case of ', this TCP frame is a signal notifying the transmitting apparatus that downlink data by downlink TCP communication has been received. There is a possibility of receiving data continuously.

  The TCP communication detection device 44a detects the start of TCP communication based on the values of the 'SYN' bit and the 'ACK' bit in the 'Control Bits' portion of the TCP header included in the upstream TCP signal, or 'ACK' 'When reception of data is detected based on the bit value, a margin value (here, 1 second) is set in the timer field 46c of the corresponding ONU (or LLID) record in the table 46. It may be simplified so that only one of the former and the latter is detected.

  When detecting the end of the downstream TCP communication based on the value of the 'FIN' bit in the 'Control Bits' part of the TCP header, the TCP communication detection device 44a clears the downstream TCP flag field 46b of the TCP communication table 46, 0 is set in the timer field 46c. As another method, the TCP communication detection device 44a may delete the record of the corresponding ONU (or LLID) in the table 46 when detecting the end of the downstream TCP communication.

  The subsequent operation is the same as that of the embodiment shown in FIG.

  Although the invention has been described with reference to specific illustrative embodiments, various modifications and alterations may be made to the above-described embodiments without departing from the scope of the invention as defined in the claims. This is obvious to an engineer in the field to which the present invention belongs, and such changes and modifications are also included in the technical scope of the present invention.

It is a schematic block diagram of one Example of this invention. It is a schematic diagram which shows the sequence from the connection of TCP to a cutting | disconnection. An example of the configuration and numerical values of the TCP communication table 46 is shown. A numerical example of the bandwidth allocation table 50 when none of the ONUs 22-1 to 22-3 is performing downlink TCP communication is shown. The example of the content of the table 50 after the downstream TCP communication toward ONU22-1 is detected is shown. The schematic diagram of the timing sequence of the upstream signal on the optical fiber 16 corresponding to FIG. 4 is shown. The schematic diagram of the timing sequence of the upstream signal on the optical fiber 16 corresponding to FIG. 5 is shown. It is a schematic block diagram of the second embodiment of the present invention. It is a schematic diagram which shows the sequence of TCP communication from a server to a client. It is a schematic diagram which shows the sequence of TCP communication from a server to a client in case a PON system intervenes. It is an example of arrangement | positioning of the transmission frame on a PON transmission line, (a) shows the case where a transmission interval is small, and (b) shows the case where it is large.

Explanation of symbols

10: Optical termination device (OLT)
10a: Uplink port 10b: PON port 12: Host network 14: Server 16: Optical fiber 18: Optical couplers 20-1 to 20-3: Optical fibers 22-1 to 22-3: Optical termination unit (ONU)
24-1 to 24-3: Computer 30: LAN interface 32: Multiplexer 34: OLT control device 36: Electrical / optical converter 38: WDM optical coupler 40: Optical / electrical converter 42: Separating device 44: TCP communication detection Device 46: TCP communication table 48: Timer 50: Bandwidth allocation table 60: Optical termination device (OLT)

Claims (20)

In a data transmission system in which a plurality of user termination devices are accommodated and each user termination device is allowed to transmit by TDMA,
TCP communication detection step (44, 44a) for detecting the presence or absence of downstream TCP communication to each user terminal device;
A control step of shortening the transmission interval permitted to a user terminal device having the downlink TCP communication according to the detection result of the TCP communication detection step while maintaining the transmittable data amounts within a predetermined reference period of each user terminal device equal to each other (34)
A transmission bandwidth allocation method comprising: The TCP communication detection step includes
A detection step (44, 44a) for detecting a start signal indicating the start of TCP communication of the downlink TCP communication and an end signal indicating the end of TCP communication;
Table update step (44, 44a) for updating the TCP communication table (46) indicating the presence / absence of downlink TCP communication of each user terminal device according to the detection result of the detection step
And
2. The transmission band allocation method according to claim 1, wherein the control step refers to the TCP communication table (46) and shortens a transmission interval permitted to a user terminal device having the downlink TCP communication.   The start signal is the 'SYN' bit in the 'Control Bits' portion of the TCP header of the downlink TCP signal, and the end signal is 'FIN' in the 'Control Bits' portion of the TCP header of the downlink TCP signal. 3. The transmission bandwidth allocation method according to claim 2, wherein the transmission band is a bit.   The start signal is the 'SYN' bit and the 'ACK' bit in the 'Control Bits' portion of the TCP header of the upstream TCP signal, and the end signal is the 'Control Bits' portion of the TCP header of the upstream TCP signal. The transmission bandwidth allocation method according to claim 2, wherein the 'FIN' bit is included. The TCP communication detection step includes
A detection step (44, 44a) for detecting a start signal indicating the start of TCP communication of the downlink TCP communication, presence of downlink data transmission by the downlink TCP communication, and an end signal indicating the end of TCP communication;
A table update step for updating a TCP communication table indicating the presence or absence of downlink TCP communication of each user terminal device according to the detection result of the detection step,
2. The transmission band allocation method according to claim 1, wherein the control step refers to the TCP communication table and shortens a transmission interval permitted to a user terminal apparatus having the downlink TCP communication. The start signal is the 'SYN' bit in the 'Control Bits' portion of the TCP header of the downstream TCP signal,
The presence of downlink data transmission by the downlink TCP communication is determined by the 'ACK' bit in the 'Control Bits' portion of the TCP header of the downlink TCP signal,
6. The transmission band allocation method according to claim 5, wherein the end signal is a “FIN” bit in a “Control Bits” portion of a TCP header of the downlink TCP signal. The start signal is the 'SYN' bit and the 'ACK' bit in the 'Control Bits' part of the TCP header of the upstream TCP signal,
The presence of downlink data transmission by the downlink TCP communication is determined by the 'ACK' bit in the 'Control Bits' part of the TCP header of the uplink TCP signal,
6. The transmission band allocation method according to claim 5, wherein the end signal is a 'FIN' bit in a 'Control Bits' portion of a TCP header of the uplink TCP signal.   The transmission bandwidth allocation method according to any one of claims 2 to 7, wherein the TCP communication table includes a timer field that defines a lifetime of a record indicating the presence of downlink TCP communication.   9. The transmission band according to claim 1, wherein the control step shortens a transmission interval permitted to a user terminal apparatus having the downlink TCP communication and shortens each transmission period. Allocation method.   The transmission band allocation method according to any one of claims 1 to 9, wherein the data transmission system is a PON (Passive Optical Network) system. In a data transmission system in which a plurality of user termination devices are accommodated and each user termination device is allowed to transmit by TDMA,
TCP communication detection device (44, 44a) for detecting the presence or absence of downstream TCP communication to each user termination device;
A TCP communication table (46) updated by the detection result of the TCP communication detection device and indicating the presence or absence of the downstream TCP communication of each user terminal device;
Control that shortens the transmission interval permitted to a user terminal device with the downlink TCP communication by referring to the TCP communication table (46) while maintaining the transmittable data amounts within the predetermined reference period of each user terminal device equal to each other. Device (34)
A transmission bandwidth allocating device. The TCP communication detection device (44, 44a) detects a start signal indicating the start of TCP communication of the downstream TCP communication and an end signal indicating the end of TCP communication, and the TCP communication table (46) is determined according to the detection result. transmitting bandwidth allocation apparatus of claim 1 1, characterized in that update.   The start signal is the 'SYN' bit in the 'Control Bits' portion of the TCP header of the downlink TCP signal, and the end signal is 'FIN' in the 'Control Bits' portion of the TCP header of the downlink TCP signal. 13. The transmission band allocation device according to claim 12, wherein the transmission band allocation device is a bit.   The start signal is the 'SYN' bit and the 'ACK' bit in the 'Control Bits' portion of the TCP header of the upstream TCP signal, and the end signal is the 'Control Bits' portion of the TCP header of the upstream TCP signal. 13. The transmission bandwidth allocation apparatus according to claim 12, wherein the 'FIN' bit is included.   The TCP communication detection device (44, 44a) detects a start signal indicating the start of TCP communication of the downlink TCP communication, presence of downlink data transmission by the downlink TCP communication, and an end signal indicating the end of TCP communication, The transmission bandwidth allocating device according to claim 11, wherein the TCP communication table (46) is updated according to the detection result. The start signal is the 'SYN' bit in the 'Control Bits' portion of the TCP header of the downstream TCP signal,
The presence of downlink data transmission by the downlink TCP communication is determined by the 'ACK' bit in the 'Control Bits' portion of the TCP header of the downlink TCP signal,
The transmission band allocation device according to claim 15, wherein the end signal is a 'FIN' bit in a 'Control Bits' portion of a TCP header of the downstream TCP signal. The start signal is the 'SYN' bit and the 'ACK' bit in the 'Control Bits' part of the TCP header of the upstream TCP signal,
The presence of downlink data transmission by the downlink TCP communication is determined by the 'ACK' bit in the 'Control Bits' part of the TCP header of the uplink TCP signal,
The end signal is transmitted bandwidth allocation apparatus of claim 1 5, characterized in that the 'FIN' bits in the 'Control Bits' portion of the TCP header of the uplink TCP signal.   The transmission bandwidth allocation apparatus according to any one of claims 11 to 17, wherein the TCP communication table includes a timer field that defines a lifetime of a record indicating the presence of downlink TCP communication.   The transmission band according to any one of claims 11 to 18, wherein the control device shortens a transmission interval permitted to a user terminal device having the downlink TCP communication and shortens each transmission period. Allocation device.   The data transmission system is a PON (Passive Optical Network) system, and the transmission band allocation device is incorporated in an optical terminal device arranged in a center station of the PON system. 2. A transmission bandwidth allocation device according to claim 1.

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